Man-made systems are assembled out of components with physical properties engineered to perform specific functions within the final assembly. By contrast, biological systems self- assemble, using genetic control to continuously regulate tissue biomechanics and tissue function throughout embryogenesis and organogenesis. Genetic analyses have revealed many of the underlying principles of pattern formation and cell differentiation during development. However, neither the biomechanics of development nor the genetic control of these biomechanics is well understood. The tailbud is the posterior leading edge of the growing vertebrate embryo and contains bipotential neural/mesodermal stem cells as well as spinal cord and mesodermal progenitors. This proposal focuses on the roles of cell-cell and cell-extracellular matrix (ECM) adhesion in defining tissue biomechanics in the extending tailbud. It is hypothesized that Fibronectn- dependent mechanical coupling between these tissues maintains parallel orientation of their posteriorly directed forces. This coupling increases the net posteriorly directed force. Within the posterior paraxial mesoderm, it is hypothesized that Fibronectin fibrillogenesi and remodeling drives tissue assembly. Tissue assembly requires integration of cell-cell and cell-ECM adhesion via Cadherin 2 which regulates Fibronectin matrix dynamics and produces a phase transition within the tissue from a viscoelastic fluid to a viscoelastic solid In Aim I-A, the lab will examine whether inter-tissue adhesion between the paraxial mesoderm and notochord promotes posterior elongation. In Aim I-B, the lab will quantify Fibronectin matrix dynamics in the paraxial mesoderm and test whether Fibronectin fibrillogenesis helps drive paraxial mesoderm elongation. In Aim II, the lab examines the roles of cadherin 2 and integrin ?5 in regulating the transition in Fibronectin matrix and cell motion dynamics during the assembly of the paraxial mesoderm. In Aim III, the lab measures the contribution of these cell and tissue level processes to the generation of posteriorly directed force in the extending tailbud.